This invention relates generally to turbine components and, specifically, to coatings applied to turbine buckets, nozzles and the like.
The so-called Dense Vertically Cracked (xe2x80x9cDVCxe2x80x9d) Thermal Barrier Coating (xe2x80x9cTBCxe2x80x9d) is a ceramic coating, and by definition, is dense, hard and difficult to abrade. Examples may be found in U.S. Pat. Nos. 6,047,539 and 5,830,586. See also U.S. Pat. Nos. 5,281,487; 5,897,921; 5,989,343; and 6,022,594. The thermal spray process (typically a plasma spray process) used to achieve the required structural characteristics (i.e., those that will produce the mechanical and thermal properties desired in the coating), however, also produces a rough surface that is aerodynamically unacceptable. The thickness control capability of this process is also less than the limits required by the design. Thus, the coating as applied must be thicker than the desired end product so that it can be mechanically abraded (xe2x80x9cfinishedxe2x80x9d) to within the required limits of both thickness and surface roughness. This operation requires manual removal of excess material with diamond-impregnated disks, and has proven to be difficult, time consuming, and expensive, often resulting in rework resulting from xe2x80x9coverfinishing,xe2x80x9d i.e., abrading to a thickness less than required.
All prior efforts that we are aware of appear to have centered around finding a more effective media (i.e., ceramics other than diamond) to use in the finishing operation.
This invention involves the creation of a thin, soft (i.e., less dense), sacrificial outer layer of the TBC that is easily removed by xe2x80x9cconventionalxe2x80x9d finishing techniques and materials. The ability to apply this thin, soft sacrificial layer of the same chemical composition enables the surface finishing operation to be performed more rapidly. Because it will be noticeably easier to remove than the fully dense layers of coating beneath it, it provides an inherent xe2x80x9cfail-safexe2x80x9d indicator. In other words, a finishing operator will be immediately aware that most of the sacrificial layer has been removed by the sudden increase in removal difficulty that will then warn that minimum thickness limits are being approached. Thus, the approach should minimize the potential for xe2x80x9coverblendingxe2x80x9d (i.e., removal of too much coating during finishing, resulting in under minimum thickness requirements). Because this soft outer layer will be easier and faster to remove, it will reduce the time and the number of diamond impregnated disks required to finish a component by approximately 50%. This technique also facilitates achieving the surface roughness requirements in that the softer outer layer will fill the surface irregularities or xe2x80x9cpocketsxe2x80x9d in the harder underlayer, thus providing a smoother surface.
Accordingly, in its broader aspects, the invention relates to a process for applying a thermal barrier coating to a machine component comprising:
a. applying a plurality of layers of the thermal barrier coating on the component, utilizing a nozzle at a first predetermined distance from the component; and
b. applying an outer layer of said thermal barrier coating on the component, with the nozzle at a second distance from the component, greater than said first distance.
In another aspect, the invention relates to a process for coating and surface finishing a machine component to provide a final coating of predetermined thickness and surface roughness comprising:
a. spraying a plurality of layers of a ceramic thermal barrier coating on the component, utilizing a spray nozzle at a first distance from the component;
b. spraying an outer layer of the ceramic thermal barrier coating on the component, with the spray nozzle at a second, greater distance from the component; and
c. abrading the outer layer to thereby remove some or all of the outer layer to achieve a predetermined final coating thickness and desired surface roughness.
In still another aspect, the invention relates to a process for applying a dense, hard, ceramic thermal barrier coating on a turbine component comprising:
a. spraying a plurality of layers of a ceramic thermal barrier coating on the turbine component, utilizing a plasma-spray torch at a first distance from the component;
b. plasma-spraying a sacrificial layer of the ceramic thermal barrier coating on the turbine component, with the plasma-spray torch at a second, greater distance from the turbine component to thereby make the sacrificial layer less dense than the plurality of layers; and
c. abrading the sacrificial layer to thereby remove some or all of the sacrificial layer to achieve a desired final coating thickness and surface roughness.